In a variety of subterranean environments, such as wellbore environments, downhole tools are used in many applications. For example, downhole tools may comprise safety valves, flow controllers, packers, gas lift valves, sliding sleeves and other tools. In many applications, the downhole tools are hydraulically controlled via hydraulic control lines. For example, a dedicated hydraulic control line may be run downhole to an individual tool. However, the number of tools placed downhole can be limited by the number of control lines available in a given wellbore. Often, the maximum number of hydraulic control lines is between two and four lines. The space constraints of the wellbore or wellbore equipment, e.g. packers, located within the wellbore also can limit the number of control lines. Even if additional control lines can be added, the additional lines tend to slow the installation and increase the cost of installing equipment downhole.
Attempts have been made to reduce or eliminate the use of hydraulic control lines through, for example, the use of multiplexers, electric/solenoid controlled valves or custom-designed hydraulic devices and tools that respond to sequences of pressure pulses. Such designs, however, have proved to be relatively slow and/or expensive. Also, in the case of custom-designed hydraulic devices and tools, two control lines can only be used to control a maximum of two tools.